The basic principle of the ultrasonic wave in a thickness measurement is similar to that of an optical wave. The ultrasonic wave pulses are transmitted by the probe face and are coupled to the object to be measured using a viscous fluid called "couplant". These ultrasonic pulses propagate through the measured object and reflect back to the transducer when they encounter the back surface. The thickness of the object is determined by precisely measuring the time the ultrasonic wave travels in the object.